Triptane is a highly branched hydrocarbon which can be used as a fuel additive for motor and aviation gasoline because of its high octane rating. It is generally produced by isomerising or reacting aliphatic, cycloaliphatic and/or alkyl aromatic hydrocarbons in the presence of an isomerisation catalyst. An example of such a process is described in U.S. Pat. No. 3,766,286. This document describes the use of reaction temperatures of −30 to 100° C. and contact times of 0.1 to 10 hours. The preferred embodiment, for instance, describes the isomerisation of n-heptane at 25° C. for 5 to 6 hours. The triptane yields obtained were between 0.4 and 1.4 wt %.